Downhole spectroscopic hydrogen sulfide detection

ABSTRACT

Methods and related apparatuses and mixtures are described for detecting hydrogen sulfide in a formation fluid downhole. A detection mixture is combined with the formation fluid downhole. The detection mixture includes metal ions for reacting with hydrogen sulfide forming a metal sulfide, and charged nanoparticles sized so as to inhibit significant aggregation of the metal sulfide so as to enable spectroscopic detection of the metal sulfide downhole. The combined mixture and formation fluid is then spectroscopically interrogated so as to detect the presence of the metal sulfide thereby indicating the presence of hydrogen sulfide in the formation fluid. The mixture also includes chelating ligands for sustaining thermal endurance of the mixture under downhole conditions.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation from U.S. patent applicationSer. No. 11/925,219 filed Oct. 26, 2007 which is incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is generally related to detection and sensing ofproperties of downhole fluids. More particularly, this patentspecification relates to downhole spectroscopic detection of substancessuch as Hydrogen Sulfide using colloidal detection mixtures.

2. Background of the Invention

Hydrogen sulfide (H₂S) occurs extensively in a number of subsurfacehydrocarbon reservoirs under anaerobic conditions. The presence ofhydrogen sulfide is highly corrosive to casing, tubing, and othermetallic and polymeric tools, an effect that is considerably acceleratedby low pH and the presence of carbon dioxide. This has a significantimpact on the overall hydrocarbon recovery processes, during whichmaterials selection and corrosion control are of great importance.Additionally, H₂S is hazardous to humans even at minute concentrationlevels (for example, about 100 ppm).

The H₂S content of reservoir fluids can be determined from samplescollected by wireline fluid Sampling tools such as Schlumberger'sModular Dynamics Tester or other sampling tools. Fluid samples areusually collected in metal containers, which are able to maintain thepressures at which the samples were collected. However, a problemassociated with sampling fluids containing hydrogen sulfide is partialloss of the gas by reaction of the metal components, particularly thosemade from iron-based metals. The hydrogen sulfide gas readily formsnon-volatile and insoluble metal sulfides by reaction with many metalsand metal oxides, and analysis of the fluid samples can therefore givean underestimate of the true sulfide content.

As a result, the in situ detection and measurement of hydrogen sulfideis widely regarded as a critical parameter needed for well completionand production strategies. Due to the high chemical reactivity ofsulfide species, various detection strategies including spectroscopy,electrochemistry, chromatography and combinations thereof have beenproposed. For example, see Wardencki, W. J. “Problems with thedetermination of environmental sulphur compounds by gas chromatography”Journal of Chromatography A, Vol 793, 1 (1998). U.S. Pat. No.6,939,717B2 describes feasible electrochemical and optical methodologiesand embodiments aimed at downhole detection of hydrogen sulfide.

SUMMARY OF THE INVENTION

The present invention relates to a mixture that is provided for use indownhole spectroscopic detection of hydrogen sulfide. The mixtureincludes metal particles for reacting with hydrogen sulfide therebyforming a metal sulfide species and nanoparticles sized so as to inhibitsignificant aggregation of insoluble metal sulfide species so as toenable spectroscopic detection of the metal sulfide species downhole.Chelating ligands are preferably included in the mixture for sustainingthermal endurance of the mixture under downhole conditions. The metalparticles are preferably metal ions.

In accordance with another embodiment of the invention, a method ofdetecting hydrogen sulfide in a formation fluid downhole is alsoprovided. A detection mixture is combined with the formation fluiddownhole. The detection mixture includes metal particles for reactingwith hydrogen sulfide forming a metal sulfide, and nanoparticles sizedso as to inhibit significant aggregation of the metal sulfide so as toenable spectroscopic detection of the metal sulfide downhole. Thecombined mixture and formation fluid is then spectroscopicallyinterrogating so as to detect the presence of the metal sulfide therebyindicating the presence and/or quantity of hydrogen sulfide in theformation fluid.

In accordance with another embodiment of the invention, a system isprovided for detecting hydrogen sulfide downhole. The system includes adetection mixture for reacting with hydrogen sulfide, a downhole mixturedelivery system for exposing the detection mixture to fluids collectedfrom a subterranean formation in a downhole setting, and an opticaldetection system for detecting the reacted mixture that indicated thepresence of hydrogen sulfide in the exposed formation fluid.

In accordance with another embodiment of the invention, a method is alsoprovided for dispersing a compound which is otherwise insoluble in asolvent into a homogeneous solution for use spectroscopic analysis of afluid comprising the step of combining the compound with the solvent andnanoparticles, wherein the nanoparticles are sized and charged so as toinhibit significant aggregation which would otherwise hinderspectroscopic analysis of the fluid.

Further features and advantages of the invention will become morereadily apparent from the following detailed description when taken inconjunction with the accompanying Drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further described in the detailed descriptionwhich follows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention,in which like reference numerals represent similar parts throughout theseveral views of the drawings, and wherein:

FIG. 1 shows a set of typical visible spectra of a homogeneous colloidaldispersion according to embodiments of the invention;

FIG. 2 illustrates a representation of lead species being confined inclose proximity, or directly on to the surface of silica nanoparticles,according to embodiments of the invention;

FIG. 3 shows the average values of three independent samples baked at150° C. for 24 hours against respective ambient temperature referencesaccording to embodiments of the invention;

FIG. 4 shows the uncertainty of the data set is presented in FIG. 3;

FIG. 5 shows four pairs of spectra of a Cd/NTA-Ludox® dispersion beforeand after baking at 150° C. for 24 hours, according to an embodiment ofthe invention;

FIG. 6 shows critical loading ratios of mixtures according toembodiments of the invention;

FIG. 7 is a table summarizing the results of thermal endurance test oflead coupled with a group of chelating ligands with different bindingcapacities, according to embodiments of the invention;

FIG. 8 shows optical absorption curves of various metal sulfides inLudux®, according to embodiments of the invention;

FIG. 9 shows an example of a flowline-based H₂S detection systemaccording to embodiments of the invention; and

FIG. 10 shows an example of a sample bottle based H₂S detection systemaccording to embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The particulars shown herein are by way of example and for purposes ofillustrative discussion of the embodiments of the present invention onlyand are presented in the cause of providing what is believed to be themost useful and readily understood description of the principles andconceptual aspects of the present invention. In this regard, no attemptis made to show structural details of the present invention in moredetail than is necessary for the fundamental understanding of thepresent invention, the description taken with the drawings makingapparent to those skilled in the art how the several forms of thepresent invention may be embodied in practice. Further, like referencenumbers and designations in the various drawings indicated likeelements.

The present invention is directed to a mixture that is provided for usein downhole spectroscopic detection of hydrogen sulfide. The mixtureincludes metal particles for reacting with hydrogen sulfide therebyforming a metal sulfide species and nanoparticles sized so as to inhibitsignificant aggregation of insoluble metal sulfide species so as toenable spectroscopic detection of the metal sulfide species downhole.Chelating ligands are preferably included in the mixture for sustainingthermal endurance of the mixture under downhole conditions. The metalparticles are preferably metal ions.

It has been found that a difficulty in prior Hydrogen sulfide (H₂S)detection methods exist due to rapid precipitation when the H₂S reactswith the metallic or other substances used for detection. Since themetal sulfide very quickly precipitates out of the detection solution,its optical detection is often very difficult or impractical. Accordingto embodiments of the invention, mixtures can be provided that have arelatively large surface area onto which the reacting sulfide canprecipitate. The surface area is provided in the form of particles thatare small enough that when optical detection methods can be used, suchas with visible light, the precipitated sulfide has the appearance ofbeing a fully solvated, although on a molecular level the sulfide is nolonger in the solution phase.

According to embodiments of the invention, a specific detection andmeasurement strategy for hydrogen sulfide can be provided based on thecolorimetric features of metal sulfide species that are dispersed into ahomogeneous colloidal dispersion in the presence of an overwhelmingamount of silica nanoparticles in an alkaline aqueous medium. Forexample, a ratio of Si 7000 mM:Pb²⁺ 3 mM has been found to be effectivein some applications. By adding an appropriate chelating ligand, themetal-ligand-silica system sustains high recovery yield after extensivethermal treatment that is representative of downhole sampling anddetecting processes. The provided mixture can be used in combinationwith a downhole optical detection system, such as Schlumberger's LiveFluid Analyser (LFA), which is operated as part of Schlumberger'sopenhole logging tool Modular Formation Dynamics Tester (MDT).Accordingly, a viable means for in situ detection and measurement ofhydrogen sulfide in the formation gas can be provided.

FIG. 1 shows a set of typical visible spectra of a homogeneous colloidaldispersion according to embodiments of the invention. In this example, amixture comprising of Pb²⁺/NTA-Ludox® and water reacted withincreasingly higher concentrations of sulfide. In general, the curvesexhibit a shoulder at around 600 nm and a peak at around 310 nm. FIG. 1shows that the sensitivity of the solution is sufficient to detectsulfide concentrations as low as around 3 ppm. Additionally, theproportional growth of the optical density with the sulfideconcentration can be used to make quantitative measurements.

The water insoluble metal sulfide should be dispersed into homogeneouslydistributed particles that are small enough to remain in the bulk of thesolution. This is preferably achieved via charge matching between themetal species and the silica particles (4). Colloidal particles thathave been found to be suitable for some applications are marketed underthe brand name Ludox®, and can be obtained from either GRACE Davison orSigma-Aldrich. The Ludox® particles are discrete uniform spheres with adiameter of about 22 nm. They have no internal surface area or apparentcrystallinity, with an apparent surface area of 150 m²/g. At a typicalpH of 9, the silica particle surface is negatively charged due todeprotonation of its surface silano groups. These particles (for examplein a solution of approximately 3.3×10¹⁶ particles/g) repel one anotherthereby yielding a stable solution, capable of confining the positivelycharged metal species either in the close proximity of, or directlyonto, their surface. Such an effect has been found to be strong enoughto prevent the following two adverse processes from happening: (1)precipitation of an otherwise poorly soluble metal hydroxide; and (2)precipitation/aggregation of an otherwise poorly soluble metal sulfidespecies into micron sized powder. This allows quantitative analysis ofsulfide using absorption spectroscopy without interference fromscattering. In general, it has been found the nanoparticles should besized between about 7 nm and 25 nm. If the nanoparticles are too small,then the limit of a colloidal particle will be reached. The particleswill become fully solvated in water which will result in the charge ofthe particles not functioning as desired. If the particles are toolarge, it has been found that the aqueous solution becomes overly milkyor opaque so as to impede spectroscopic analysis.

FIG. 2 illustrates a representation of lead species being confined inclose proximity, or directly on to the surface of silica nanoparticles,according to embodiments of the invention. On the left side of FIG. 2,lead ion 208 is in close proximity to silica nano particles 210, 212,214 and 216. On the right sides of FIG. 2, lead sulfide 220 is shown inclose proximity to silica nanoparticles 210, 212, 214 and 216. Theconfinement of the lead species by the silica nanoparticles could be ananocage function or similar to a nanocage as shown in FIG. 2. However,the arrangement and functioning on the molecular level may not beprecisely understood at present.

In selecting a suitable mixture, the thermal endurance of the reactionsystem should be considered in a temperature regime analogous to theanticipated wellbore conditions. For many applications, a temperature of150° C. is suitable and is used throughout this description unlessotherwise indicated. It was observed that a 24-hour baking of thePb²⁺-Ludox® binary mixture resulted in a considerably lower yield ofoptical density as compared to the ambient temperature counterpart. Atthe end of the baking period, the sample was cooled down to ambienttemperature. According to an aspect of the invention, an appropriatechelating ligands to be selected tends to shield Pb²⁺. A particularlyeffective ligand, nitrilotriacetic acid (NTA), was identified thatconsistently has the recovery yield in optical density better than 90%after 24 hour baking at 150° C.

FIG. 3 shows the average values of three independent samples baked at150° C. for 24 hours against respective ambient temperature references.FIG. 3 shows the absorbance versus sulfide concentration relationship atthree discrete wavelengths of 815 nm, 680 nm and 570 nm. Each paircompares the ambient temperature sample to its counterpart throughbaking at 150° C. for 24 hours, and then probed at ambient temperature.For example, the mixture tested had the following characteristics:Pb(NO₃)₂ 4.03 mM, NTA 6.14 mM, Ludox® 35%, pH 9. FIG. 3 shows that thesamples exhibit a nearly 100% recovery yield in optical density in allthe three wavelengths at sulfide concentrations up to 89 ppm (withexception for 680 nm at this point). This concentration of sulfideapproaches the stoichiometric ratio of unity with lead (theoreticalthreshold at 103 ppm sulfide). For sulfide concentrations in excess ofthose of lead ion the optical density is constant, indicating thatsaturation of the measurement has been reached.

In general, the pH of the mixture should be carefully selected. If thepH is too low, for example below 7.5, problems can occurs such asgelling of the Ludux® and/or multiple species of H₂S occurring and themeasurement may lose quantification. If the pH is too high, for exampleabove 10.5, the silica particles will start to dissolve in the aqueoussolution. Additionally, corrosion of the downhole tool and casing becomea problem at high pH levels. It has been found that for manyapplications a pH range of 9.0-9.5 can be ideal.

Referring to FIG. 4, it has been found in general, that shorterwavelengths offer higher confidence in data accuracy. FIG. 4 shows theuncertainty of the data set is presented in FIG. 3. For example, at 570nm, the largest uncertainty at the 7 ppm level, which is 9.6%representing a possible error of ±0.67 ppm. While the smallestuncertainty is at the 89 ppm level, which is 1.9% representing apossible error of ±1.7 ppm. These levels have been found to be wellwithin tolerance of many intended applications.

FIG. 5 shows four pairs of spectra of a Cd/NTA-Ludox® mixture before andafter baking at 150° C. for 24 hours, according to an embodiment of theinvention. For example, the mixture characteristics were: Cd(NO₃)₂ 2.5mM, NTA 5.1 mM, Ludox® 32%, pH 9. These spectra exhibit a shoulder ataround 430 nm, and a peak at 315 nm that red-shift to ˜350 nm as aresult of baking. This cadmium system also affords high recovery yieldin optical density across the sulfide concentration range from 18 to 71ppm, representing a viable alternative to lead.

Referring to FIG. 6, according to an aspect of the invention, it ispreferable not to exceed a critical loading ratio of a particularmixture. The critical loading ratio can be defined herein as the amount(in mM) of metal ions which can be combined with a particularnanoparticle-water weight ratio, beyond which there is a significantrisk of a large scale precipitation that destroys the homogeneity of themixture. FIG. 6 shows critical loading ratios of mixtures according toembodiments of the invention. FIG. 6 shows three domains for aqueousmixtures of Pb²⁺/NTA in Ludox® all after baking at 150° C. for 24 hours:(1) below 25%, the Ludox® is not sufficient to accommodate the normalamount of Pb²⁺/NTA; (2) between 32-45%, the Ludox® is functioning finewith [Pb²⁺] up to 7 mM; and (3) at even higher concentrations, theLudox® tends to gel as a result of baking. Thus, after being baked at150° C. for 24 hours, a Ludox® concentration greater than 45% makes themixture liable to gel formation. On the other hand, at Ludox®concentrations lower than 20%, its binding capacity is not strong enoughto accommodate the given amount of Pb²⁺/NTA.

FIG. 7 is a table summarizing the results of thermal endurance test oflead coupled with a group of chelating ligands with different bindingcapacities, according to embodiments of the invention. The followingabbreviations are used in FIG. 7: NTA-nitrilotriacetic acid, CA-citricacid; EDDA-ethylenediamine diacetic acid; IDAA-iminodiacetic acid;PDCA-pyridinediacetic acid; IDMPA-iminodimethyl phosphorous acid. Asshown in FIG. 7, NTA and IDMPA represent two chelating ligands for leadthat provide a high level of thermal endurance and remain chemicallyactive to sulfide.

FIG. 8 shows optical absorption curves of various metal sulfides inLudux®, according to embodiments of the invention. Experiments in Ludox®solutions have shown that Nickel, Copper and Selenium can be used for inmixtures for detecting H₂S, in addition to Lead and Cadmium as describedabove. Copper and Selenium, in particular, give rise to a specificabsorption peak which provides a clear distinction between absorbancedue to the particles from scattering and absorbance due to oil or othercomponents. Furthermore, other metals such as Cobalt, Silver and Tincould be useful in detection mixtures under some circumstances.

As has been described herein, the silica particles in Ludox® aredissolved in water. However, according to other embodiments of theinvention, other solvents can be used with the detection mixtures. Forexample, the water can be replaced by formamide. This is done by theaddition of formamide to the Ludox® and heating the solution to about110 degrees C. The water will evaporate but the silica will remainsuspended in the formamide. Formamide is an organic polar solvent. Thesilica suspension in formamide can be used with the detection mixturesas described herein. The formamide suspension offers some advantagesover water. For example, formamide has a boiling point of 220 degrees C.which minimizes the risk of evaporation of the solvent. In the case ofLudox® the water will evaporate at 100 degrees C. causing the solutionto gel, which could cause problems the tool when used downhole.Furthermore, the silica particles dissolved in formamide are insensitiveto the salt concentration whereas the aqueous Ludox® mixture can tend todevelop into a gel when in contact with higher concentrations salts.Higher loads of lead or other metal particles can be used with solventssuch as formamide since such mixtures have less of tendency to gel.However, one disadvantage of silica particles in formamide can be arelatively rapid discoloring of the solution in formamide when exposedto sulfide. The coloring tends to disappear with time. Depending on thesulfide concentrations the color completely disappears within a fewhours. According to other embodiments of the invention, other solventscan be used instead of water or formamide. When selecting an appropriatesolvent, an organic solvent having relatively high polarity can besuitable, such as dimethylformamide, glycol.

As mentioned, mixtures described according to the foregoing embodimentsof the invention can be used in combination with a downhole opticaldetection system, such as Schlumberger's Live Fluid Analyser (LFA),which is operated as part of Schlumberger's openhole logging toolModular Formation Dynamics Tester (MDT). FIG. 9 according to aspects ofthe invention, shows two examples of systems for optically detecting andmeasuring H₂S downhole using the mixtures as described herein. FIG. 9shows an example of a flowline-based H₂S detection system according toembodiments of the invention. The flow line 910, carries test fluidsfrom the formation, from the left side of flow line 910 in FIG. 9, to aborehole or to one or more sampling chambers, on the right side of flowline 910 in FIG. 9. The flow line 910 receives an amount of a suitablemixture from bottle 912. The flow of the mixture stored in bottle 912into flow line 910 is controlled by an electro-mechanical valve 914. Themixture mixes with the fluids in the flow line 910 before reaching aninterrogation window 920 down-stream. Window 920 is preferably made froma rugged material such as sapphire to withstand an aggressive flowwithin flow line 910. A light source 916 provides light 922 either in abroad spectrum of white light or at an appropriate wavelength (e.g. 815nm, 680 nm or 570 nm, which is compatible with Schlumberger's LFA systemin Schlumberger's MDT). Light 922 enters the flow line 910 via window920 and illuminates the fluid in flow line 910. A suitable filter 926and lens 928 aid in the detection of the signal from flow line 910through window 920 by optical detector 930. Filter 926 is designed toonly pass a narrow range of frequencies centered around the targetwavelength (e.g. 815 nm, 680 nm or 570 nm). Relating to an aspect of theinvention, in particular, absorption spectroscopy, detector 930 measuresthe optical density at the appropriate wavelength, which is thencompared with a predefined calibration curve to establish the existenceor determine the quantity of H₂S in the formation fluid. The describedoperation is then repeated for every measurement. Alternatively, lightsource 918 can be used to provide light 924 into the flow line 910.Although FIG. 9 shows the window at one side of the flow line, accordingto a different embodiment of the invention, the measurement can be madewith separated windows for illumination and detection, which can belocated at opposite sides of the flow line 910.

FIG. 10 shows according to an aspect of the invention, an example of asample bottle based H₂S detection system. Sample bottle 1010 is housedin a downhole fluid sampling tool (not shown) such as Schlumberger'sMDT. Prior to use, sample bottle 1010 is filled with the H₂S detectionmixture as described herein. During the sampling process downhole, theformation fluid is drawn into the sample bottle 1010 via inlet 1012 bycreating an appropriate pressure gradient along the flow line. Formationfluid in a gas state is drawn or bubbled into the sample bottle 1010thereby exposing the sampled formation fluid to the detection mixture inbottle 1010. Diffusion will act to mix the formation fluid with thedetection mixture so as to generate a detectable signal. However, ifaccording to an aspect of the invention, it is not be practical to waitfor sufficient diffusion to occur, such that a mixing system 1040 can beprovided. Mixing system 1040 can be based on an acoustic transducer,such as described in U.S. Pat. No. 6,988,547, or in U.S. Pat. No.6,758,090, both incorporated by reference herein. Optical detectionsystem 1014 is provided on sample bottle 1010 which can be similar tothe LFA system in Schlumberger's MDT tool. In particular, two lightsources 1016 and 1018 provide light 1022 and 1024 respectively either ina broad spectrum of white light or at an appropriate wavelengths (e.g.815 nm, 680 nm and 570 nm). The signal from sample bottle 1010 isdetected by light detector 1030 via window 1020, preferably made ofsapphire, filter 1026 and lens 1028. Filter 1026 is designed to onlypass a narrow range of frequencies centered around the target wavelength(e.g. 815 nm, 680 nm or 570 nm). Detector 1030 measures the opticaldensity at the appropriate wavelength, which is then compared with apredefined calibration curve to establish the existence or determine thequantity of H₂S in the formation fluid.

Still referring to FIG. 10, and according to another embodiment of theinvention, the sample bottle 1010 is filled with an H₂S detectionmixture, and the formation fluid is drawn into bottle 1010 as describedabove. However, the combined mixture and formation fluid is then pushedback out into a flow line such as flowline 910 in FIG. 9 where theoptical interrogation can occur using a detection system as shown inFIG. 9. According to this embodiment of the invention, optical detectionsystem 1014 need not be provided. Additionally, the mixing system 1040can be used to sufficiently mix the detection mixture and the formationfluid. If sufficient mixing occurs by the drawing in and pushing outprocess of the formation fluid, as well as diffusion in bottle 1010 andflowline 910, then mixing system 1040 need not be used.

Referring to FIGS. 9 and 10, and according to another embodiment of theinvention, show that fluorescence spectroscopy can be used to detect thereacted metal sulfide species in flowline 910 of FIG. 9 or bottle 1010of FIG. 10. Since many metal sulfide species exhibit strongfluorescence, one or more of the light sources 916, 918, 1016 and 1018can be used as an excitation energy source, and the florescence from themetal sulfide species can be detected using filters and detectors 926,1026 and 930, 1030 respectively. Existing downhole fluorescencedetection technology can be used such as used in the Composition FluidAnalyzer module to MDT tester suite provided by Schlumberger.Fluorescence spectroscopy can be especially useful when theconcentrations of metal sulfide are relatively high, due to an increasedfluorescence signal relative to background noise.

Whereas many alterations and modifications of the present invention willno doubt become apparent to a person of ordinary skill in the art afterhaving read the foregoing description, it is to be understood that theparticular embodiments shown and described by way of illustration are inno way intended to be considered limiting. For example, although thetarget substance in the embodiments described has been H₂S, it has beenfound that some thiols such as CH₃SH, C₂H₅SH and C₃H₇SH can also bedetected using the embodiments described herein. In another example,although silica based nanoparticles have been described in theembodiments herein, alternative materials such alumina (Al₂O₃), titania(TiO₂), cerium oxide (CeO₂) or other metal oxides can be use for thenanoparticle material. In another example, uncharged metal particles canbe used instead of ions. Specifically, elemental metal particles can bemanufactured or purchased having a size domain of 5-200 nm and zerosurface charge.

Further, while the present invention has been described with referenceto an exemplary embodiment, it is understood that the words, which havebeen used herein, are words of description and illustration, rather thanwords of limitation. Changes may be made, within the purview of theappended claims, as presently stated and as amended, without departingfrom the scope and spirit of the present invention in its aspects.Although the present invention has been described herein with referenceto particular means, materials and embodiments, the present invention isnot intended to be limited to the particulars disclosed herein; rather,the present invention extends to all functionally equivalent structures,methods and uses, such as are within the scope of the appended claims.

What is claimed is:
 1. A mixture for use in downhole spectroscopicdetection of hydrogen sulfide, the mixture comprising: a formation fluidcomprising hydrogen sulfide; metal ions for reacting with hydrogensulfide within the formation fluid thereby forming an insoluble metalsulfide species; and a colloidal dispersion comprising a solvent and anoverwhelming amount of nanoparticles, wherein a ratio of nanoparticlesto solvent by weight is between about 25% and about 45%; and wherein theinsoluble metal sulfide species are dispersed into the nanoparticles,thus resulting in a stabilized homogeneous colloidal liquid mixture,that inhibits significant aggregation of the insoluble metal sulfidespecies, and provides for spectroscopic detection of the metal sulfidespecies downhole.
 2. A mixture according to claim 1, further comprising:chelating ligands for sustaining thermal endurance of the mixture underdownhole conditions; and wherein the solvent is water into which themetal ions and the nanoparticles are dissolved, and wherein absorptionspectroscopy is used for the detection of the metal sulfide species. 3.A mixture according to claim 1, wherein the solvent comprises an organicpolar solvent into which the metal ions and nanoparticles are dissolved.4. A mixture according to claim 1, wherein the metal ions are ions fromone or more metals selected from the group consisting of Lead, Copper,Selenium, Nickel, Cadmium, and Tin.
 5. A mixture according to claim 2,wherein the chelating ligands include one or more types of ligandsselected from the group consisting of nitrilotriacetic acid,iminodimethyl phosphoric acid, and iminodiacetic acid.
 6. A mixtureaccording to claim 1, wherein the homogeneous colloidal dispersioncomprises an aqueous solution having a pH range between about 9.0 andabout 9.5.
 7. A mixture according to claim 1, wherein the nanoparticlesare silica based and sized between about 7 nm and about 25 nm.
 8. Amixture according to claim 1, wherein fluorescence spectroscopy is usedfor the detection of the metal sulfide species.